A seamless tube can be produced by the Mannesmann tube-making process. This tube-making process comprises the following steps:
(1) piercing-rolling a starting material (round billet), which is heated to a predetermined temperature, by a piercing machine to deform the same into a hollow blank (hollow shell);
(2) elongation-rolling the hollow blank by a elongation-rolling mill (for example, a mandrel mill); and
(3) sizing-rolling the hollow blank, which has been subjected to elongation-rolling, by a sizing mill (for example, a stretch reducer).
FIG. 1 is a schematic diagram to illustrate the piercing-rolling of a starting material by a conventional piercing machine. As shown in the drawing, the piercing machine includes a pair of conical rolls 4 each of which is inclined with respect to a pass line PL, a bullet-shaped plug 100 as a piercing tool, and a mandrel 3 that is coupled with the rear part of the plug 100. A starting material 7 is fed in an axial direction while being rotated in a circumferential direction by the conical rolls 4. At this time, the starting material 7 is pierced in the central portion thereof by the plug 100 to form a hollow blank 8.
During piercing-rolling by the piercing machine, flaws may occur in the inner surface of the hollow blank (hereafter, referred to as “inner surface flaws”). The principal mechanism of the occurrence of inner surface flaws is as follows. A rotary forging effect during piercing causes Mannesmann fracture to occur in the central portion of starting material on the upstream side of the front edge of plug. The resultant Mannesmann fracture is subjected to shear strain in a circumferential direction by the conical rolls and the plug. As a result, the Mannesmann fracture propagates in a circumferential direction to grow into inner surface flaws.
In order to suppress the occurrence of inner surface flaws due to the Mannesmann fracture, it is effective to reduce the friction coefficient of the plug surface. The reason is as described below. Reducing the friction coefficient of the plug surface will increase the feeding speed of the starting material to be pierced, thereby suppressing the rotary forging effect. Moreover, reducing the friction coefficient of the plug surface will suppress the shear strain in a circumferential direction. The suppression of the rotary forging effect and the shear strain can prevent the development of the Mannesmann fracture, thereby enabling the suppression of the occurrence of inner surface flaws.
The reduction of the friction coefficient of the plug surface also contributes to the prevention of the wear and melting loss of the plug. Accordingly, it becomes possible to prevent the formation of concavo-convex irregularities on the plug surface, and also to suppress the occurrence of inner surface flaws due to the concavo-convex irregularities.
Conventional arts for reducing the friction coefficient of the plug surface include the followings.
Patent Literatures 1 and 2 disclose a method of piercing-rolling in which, using a plug provided with an ejection hole that opens in the front edge of the plug, piercing-rolling is performed while a lubricant is injected from the ejection hole. However, the front edge of the plug disclosed in those Patent Literatures 1 and 2 comes into contact with the starting material at a high interfacial pressure. For that reason, in order to inject the lubricant from the ejection hole that opens in the front edge of plug, it is necessary to inject the lubricant at a pressure higher than the deformation resistance of the starting material in contact with the front edge of plug. Further, the opening of the ejection hole may be deformed and clogged due to the contact with the starting material.
Patent Literature 3 discloses a method of injecting a lubricant from a plug without using a highly pressurized lubricant.
FIG. 2 is a longitudinal sectional view of the plug disclosed in Patent Literature 3. As shown in the drawing, the plug 101 disclosed in Patent Literature 3 includes a front edge portion 102 having a convex curvature, a cylindrical portion 103 having a constant outer diameter, and a trunk portion 104 having an outer diameter that gradually increases toward the rear edge thereof. The ejection hole 105 opens at the front part of the trunk portion 104, adjacent to the cylindrical portion 103. When piercing-rolling is performed by using the plug 101, a gap 60 is formed between the surface of the cylindrical portion 103 and the starting material 7. Patent Literature 3 states that the gap 60 prevents the opening of the ejection hole 105 from being clogged, allowing the supply of a predetermined amount of lubricant oil. However, this plug 101 has the following problems.
During piercing, the starting material 7 may come into contact with an upper portion of the opening 105a of the ejection hole 105. This is because the ejection hole 105 opens at the front part of the trunk portion 104, adjacent to the cylindrical portion 103. When the starting material 7 comes into contact with the opening 105a of the ejection hole 105, inner surface flaws may occur in the hollow blank 8, or the opening 105a may undergo melting loss and may be clogged.
During piercing, the starting material 7 comes into contact with the vicinity of the opening 105a of the ejection hole 105 in the trunk portion 104. In association with the contact, the temperature at the opening 105a of the ejection hole 105 is raised to an elevated temperature by the heat retained by the starting material 7. Therefore, when a glass-based lubricant is used, the lubricant becomes high temperature in the vicinity of the opening 105a during piercing, leading up to the evaporation of water and the emergence of a glass component. Thus, it may happen that the glass component solidifies in the vicinity of the opening 105a when the plug is cooled after piercing, thereby causing the ejection hole 105 to be clogged.
Patent Literature 4 discloses a method of solving the problem of the plug disclosed in Patent Literature 3 described above.
FIG. 3 is a diagram to illustrate the plug disclosed in Patent Literature 4, whereas FIG. 3A is a longitudinal sectional view of the plug, and whereas FIG. 3B is a longitudinal sectional view to illustrate how piercing-rolling undergoes. As shown in FIG. 3A and FIG. 3B, the plug 120 disclosed in Patent Literature 4 is configured such that a cylindrical portion 122 is provided between a front edge portion 121 and a trunk portion 123, and an ejection hole 124 opens on the surface of the cylindrical portion 122. When piercing-rolling is performed using the plug 120, a gap 60 is formed between the starting material 7 and the surface of the cylindrical portion 122 as shown in FIG. 3B. The gap 60 prevents the opening 124a of the ejection hole 124 from contacting the starting material 7 during piercing. Therefore, it is possible to prevent the occurrence of inner surface flaws due to the contact between the starting material 7 and the opening 124a of the ejection hole 124, and also to prevent the opening 124a from suffering melting loss and being clogged.
During piercing, the rise of the temperature at the opening 124a of the ejection hole 124 is suppressed. This is because the ejection hole 124 does not open at the front edge portion 121 or the trunk portion 123, either of which is in contact with the starting material 7. In this way, even when a glass-based lubricant is used, it is possible to inhibit the lubricant from solidifying in the vicinity of the opening 124a of the ejection hole 124, and to prevent the ejection hole 124 from being clogged by the solidified lubricant.
In the meanwhile, the plug disclosed in Patent Literature 4 described above, which has an ejection hole for injecting a lubricant, is required to have a long life since it is intended to repetitively be used for piercing. Due to this requirement, a coating of oxide scale is usually formed on the plug surface for the protection of the plug base metal (see, for example, Patent Literatures 5 to 8). The scale coating serves to insulate the heat transfer from the billet to the plug base metal and to prevent seizing between the billet and the plug during piercing. In this way, damage and melting loss of the plug base metal is suppressed and it is expected that the life of plug extends.